Right this second, after being on for a couple of weeks (and everything I'm normally running is on) - WinXP here is using about 630 megs of ram, and 710 megs of pagefile (with a peak of 900 megs - if I start playing with gfx that goes up another 700 megs ;-)
I'm running 780 threads on 40 processes, and the kernal (ie, core) is using 95 megs of the ram.
I'm also running WinUAE with 256 megs of ram allocated to it (and it actually runs a lot faster than my old 060 amiga - I'm on an athlon xp2600 - sounds like the slow WinUAE isn't using JIT).
On my old Amiga I used to have 128 megs ram (as opposed to the 1024 megs in the pc), which cost about the same in total, though there were a few years between purchases ;-)
Think my amiga tended to sit around the ~40 meg mark too, with web ib, webserver (thttpd), amirc,, rc-ftpd, etc all running... in fact, apart from the web side (which xp now handles) WinUAE still has the same things sitting on it ;-)
So all in all, xp might use more memory, but at the same cost... until you start going on about electric bills and energy usage ;-)
Robin